1. Field of the Invention
This invention relates to a power steering system with an engine idling-up mechanism for increasing the idling rotational speed of an engine in response to operation of the power steering system.
2. Description of the Prior Art
In the case of motor vehicles with a power steering system, it is necessary to provide means for preventing knocking or engine stops due to a drop of the engine speed caused by an increase of engine load during operation of the power steering system. For this purpose, it has been the conventional practice to provide, in an oil conduit interconnecting a power steering system 1 and a hydraulic pump 2, an air control valve as shown in FIG. 1, which is actuated in response to a pressure increase resulting from operation of the power steering system 1. Besides, there is provided a negative pressure actuator valve 6 which increases the open area of the throttle valve 5 of the carburetor. A rear chamber 6a of the negative pressure actuator 6 is communicated with the intake manifold 7a of an engine 7 on the downstream side of the throttle valve 5 through the air control valve 3, thereby increasing the open rate of the throttle valve 5 during operation of the power steering system to raise the engine output.
However, a difficulty is encountered in promptly increasing the engine output due to delays in operation of the negative pressure actuator 6 and engine 7. When a steering wheel is turned from a center position, the engine output can be increased in time as long as the increase of pump load resulting from operation of the power steering system 1 is smooth. In the event steering operation is once paused when a piston 9 of a power cylinder 8 is in the proximity of its stroke end, it becomes difficult to increase the engine output in time upon recommencement of the turning effort on the steering wheel which moves the power piston 9 to the stroke end, as a result causing knocking or an engine stop.
This is because, once the turning effort on the steering wheel is stopped, it puts the power steering system 1 in non-operating state, cancelling the idling-up of the engine 7. Accordingly, even if the air control valve 3 is actuated in response to the recommencement of the steering operation, it is often the case that the power piston 9 reaches the stroke end and increases the pump load before completing the boosting of the engine output, inviting instabilization of the engine rotation or an engine stop due to a drop of its rotational speed.